1. Field of the Invention
The present invention generally relates to an apparatus and method for testing a device wafer having a plurality of devices formed thereon, and more particularly, to an interface for connecting a probe card of a testing system to device pads on a device wafer.
2. Description of the Related Art
In conventional semiconductor manufacturing, a plurality of integrated circuits or devices are formed on a semiconductor substrate or wafer, and after completing the fabrication processes, the devices formed on the wafer may be cut from the wafer and packaged into individual chips, which are then tested individually to ensure that each chip performs according to certain specification. To reduce costs associated with testing chips individually, the devices formed on the wafer may be tested prior to being separated into individual chips. Conducting tests on devices formed on a wafer improves cost efficiencies generally and also facilitates sales of devices on a wafer level (i.e., sales of device wafers).
Currently, to test a wafer having a plurality of devices formed thereon, a probe card having a plurality of probe needles is utilized to provide physical contact with a plurality of device pads of the devices formed on the device wafer. Device pads are electrically conductive pads which are connected to the leads (e.g., input, output, reference voltage, ground, etc.) of the devices. In conventional testing systems, the probe needles on the probe card are precisely manufactured to match the device feature sizes and/or the corresponding pitch between device pads. As device feature sizes decrease with improved semiconductor processing techniques, the precision requirement for the probe needles becomes more stringent (e.g., smaller sizes and smaller tolerances), and the direct cost for manufacturing the probe cards/needles increases tremendously because of the strict precision requirement.
Associated with the high cost for manufacturing precision probe needles is a longer lead time needed to manufacture the probe needles. Unless the orders for the probe card are placed far in advance, costly delays between production and testing of the device wafers are likely to occur, which may result in missed market opportunities and/or loss of first product status. However, if an order was placed too far in advance, the probe card may be in production stages that cannot incorporate changes corresponding to additional changes (after the probe-card has been ordered) to the design of the devices to be formed on the wafer.
Furthermore, the probe card and probe needles manufactured for one specific device design on a wafer are generally incompatible (i.e., not reusable) for other device designs with different device sizes or leads. Although the probe needles may be re-worked in some instances to allow some reusability, re-working the probe needles is a slow, tedious and relatively expensive task, and the re-worked probe needles provide lower quality and may become unreliable because of the re-work process.
Another problem encountered in conventional testing systems is that a testing system having one probe card cannot test all devices on a wafer all at once and requires sequential testing of groups of devices, resulting in prolonged testing time and reducing throughput. Because a probe card provides a limited number of probe needles corresponding to the number of available test channels on the tester or testing equipment, the devices formed on the wafer are divided into a plurality of groups for testing purposes. Each group of devices may be tested utilizing one probe card, and the testing processes are repeated for each group of devices. Because all of the devices on the wafer cannot be tested at the same time (e.g., cannot be tested with the same touchdown), some tests may require excessively long time periods to complete, particularly with tests such as burn-in tests which may require days to complete for all devices on a wafer. Such excessive time requirements for performing device testing substantially reduces production efficiencies and throughput. Furthermore, a probe card may require an excessive number of touchdowns on the device pads on the wafer, which may result in damages to the devices formed on the wafer.
Another problem associated with testing of device wafers relates to the increasing design-for-test (DFT) features included on-chip for testing and manufacturing. DFT features are components or circuits which are designed and utilized specifically for testing the devices, but generally not utilized in normal operation of the devices. As more DFT features are included on-chip, the area or space required to implement the on-chip DFT features increases along with the additional design effort required to accommodate all components of the device in a limited substrate area. Also, the performance of the device may be hindered due to the on-chip DFT features. Thus, the additional effort required for implementing the DFT features on-chip have resulted in a reduction in the savings and utility of having the DFT features on-chip.
Current solutions to reduce design effort needed to incorporate the DFT features into a new device design are based on re-using previously designs of DFT features. However, substantial efforts from one or more DFT designers and layout designers are required to make adjustments to incorporate the previously designed DFT features into a new device design and run full verification to ensure that no mistakes were made in re-using previously designed DFT features. Another current solution for incorporating increased DFT features minimizes the required substrate area for the DFT features by providing more efficient layout or feature design. However, this is usually done as an after-thought because priority to optimize DFT features is low as compared to the need to optimize the main device performance.
In addition to the substrate area consumed by the components for implementing the DFT features, a significant proportional amount of metal signal routing area is also required by the DFT features. This becomes especially critical in areas where congestions for routing normal operation signal already exist, resulting in increased layout time, weakened power rails and/or weakened device operation signal. The performance impact to normal operation is usually minimized by isolating DFT blocks with pass gates or logic gates. However, the performance impact to normal operation has also become difficult to control as the number of DFT features grows and complexity increases.
As sales of devices at the wafer level become more prevalent, the probe card cost has also become an increasingly important part of the test cost. Therefore, there exists a need for an apparatus and method which reduce the precision requirement of the probe cards utilized in testing systems for conducting wafer level tests. There is also a need for an apparatus and method which allow a single probe card design to be utilized for testing a variety of device designs. Additionally, there is a need to reduce or minimize the number of touchdowns on device pads required to complete testing of the devices on the wafer. Furthermore, there is a need to improve efficiencies in both testing time and probe card manufacturing cost. There is also a need for an apparatus and method for reducing the production test costs by increasing throughput per tester. Also, there exists a need for an implementation of the DFT features that minimizes the impact on the device performance and reduces the design efforts required to incorporate DFT features.